Digital detector of X-rays is an apparatus comprising a radiographic screen transforming the X-ray radiation into the visible one, an optical transfer system for transmitting an image to the photosensitive CCD-matrix (a group of photodetectors based on charge-coupled devices), which converts the image projected thereon into the digital signal. From the photosensitive CCD-matrix the digital signal passes into the image processing unit. Diagnostics of an investigated object is performed using the captured and appropriately processed image, therefore the quality of captured radiographic images is the most important parameter of such systems. The self-diagnostic system of image sharpness setting makes it possible to bring down the risks of wrong diagnosis based on the captured radiographic image by means of excluding usage of an X-ray detector showing inadequate image sharpness. Such fault is possible, for instance, at excessive mechanical loads experienced by a X-ray detector at incorrect transportation or improper operating methods.
A self-testing system is known (patent CA2412711, MΠκA61B6/04, published 23, May 2003, equivalent patent U.S. Pat. No. 6,869,218) of image sharpness of a digital X-ray detector containing a radiographic screen installed in the radiation pathway from the radiographic source, and an optical transfer system for transmitting images from the screen to the photosensitive matrix of a charge-coupled device (hereafter referred to as CCD-matrix), whose output is connected to the image processing unit. The system incorporates test structures, which are located at the periphery of the screen emitting surface and outside its working zone, i.e. outside the zone of the images to be investigated, and made of a material that passes short ultraviolet waves. On the side of screen emitting surface there is a source of ultraviolet radiation of the given spectrum, which irradiates the area of the radiographic screen with the test structure. Having reached the radiographic screen, ultraviolet radiation is transformed into the visible one, which forms an image of the given test structure. When calibrating the X-ray detector, the reference images of test structures are registered and saved for the sake of comparing these with the images captured during operation.
With this system, sharpness of the captured image can be controlled only on condition of using the specified screen types having appropriate ultraviolet radiation absorption bands and appropriate radiation spectrum. Thus, choice of the radiographic screens that can be employed in these X-ray detectors is limited.
At partial coincidence of radiographic screen excitation spectra in the screen emission ultraviolet (hereinafter UV) range and the X-ray detector sensitivity in the UV range it is necessary to ensure radiation filtration in order to separate the reflected UV radiation from the secondary UV emission of the radiographic screen. This imposes additional requirements to optics and, consequently, entails a more complicated and more expensive construction. Application of these filters leads to a reduced input signal and, thus, requires increased dosage rate during radiographic investigations.